Interaction of L-malic acid with alkaline metals and open chain polyammonium cations in aqueous solution

This work reports a potentiometric, calorimetric and spectropolarimetric ul traviolet circular dichroism (UV/CD) study of the interaction of L-malic ac id with alkaline metals or (poly)ammonium (methylamine, ethylenediamine, di ethylenetriamine, triethylenetetramine, spermine, tetraethylenepentamine an d pentaethylenehexamine) cations. Stability data (log K, DeltaG degrees) we re obtained potentiometrically for the L-malic acid with (poly)ammonium cat ions systems: calorimetric measurements (25 degreesC) made it possible to o btain DeltaH degrees and T DeltaS degrees values for the complexes formed i n the systems under examination, log K values calculated (for the reaction: H(i)A(i+) + HjL(j-z) = ALH(r)((i+j-z)), with r = i + I range between 0.8 a nd 4.6, i.e., the interactions are from weak to fairly strong while maximum stability for each system is given by the species with the highest z(anion ) X z(cation) (= = charge) value. Enthalpy changes associated with reaction s H(n)A(n+) L-2 = ALH(n)((n-2)) and H(n)A(n+) + HL- = ALH(n+1)((n-1)) are a lways positive and increase progressively with n. The same is valid For T D eltaS degrees values, which indicate that these species are entropically st abilized, as expected for electrostatic interactions. It was verified that the UV/CD signal depends on both ionic medium and ionic strength value: for comparison, we used the L-malic acid signal recorded in tetramethylammoniu m chloride as baseline background salt las in potentiometry). UV/CD spectra were recorded for solutions containing both cationic and anionic species. When the cation was a protonated polyamine, CD spectra calculations were pe rformed for most stable ion pairs: the results show remarkable differences in Delta epsilon: (dm(3) mol(-1) cm(-1)) values at 205 nm (which is the L-m alate UV/CD lambda (max)) between the chiral ligand and its complex with a polyamine. (C) 2001 Elsevier Science B.V. All rights reserved.